ADSL and VDSL modems transmit and receive data using a twisted pair telephone wire. The ADSL/VDSL data connection is established between a customer premise installed modem and a central office modem. At the customer premise (CP), the twisted pair used as the ADSL/VDSL communication channel can also serve as the same channel used for voice communication, commonly known as Plain Old Telephone Service (POTS). Since the channel is shared between the two separate services, it is desirable to isolate one from the other such that interference is not coupled into the other service.
The device which provides this isolation is known as an ADSL/VDSL micro filter or splitter. The purpose of this device is to separate the POTS voice channel from the ADSL/VDSL data channel. In the case of a micro filter, the device is typically installed between the wall plate and the POTS telephone. In general, any telephone which shares the data connection with the ADSL/VDSL modem must have a micro filter installed to prevent interference from being heard while using a voice telephone. The device itself is typically a simple low-pass filter which passes low frequencies, those less than approximately 4 KHz and blocks high frequencies, those greater than 4 KHz.
A common issue with this type of arrangement is that a customer often incorrectly installs the micro filter, and puts the filter on the line connecting the wall to the ADSL/VDSL modem, thus rendering it useless since frequency bands used by the ADSL/VDSL modem are completely blocked by the micro-filter. It is therefore desirable to detect this condition and resolve it without having the customer make a service call to the ADSL/VDSL service provider.